EP0106129A2 - Expandable polystyrene particles - Google Patents

Abstract

Particulate, blowing agent-containing polystyrene with improved expandability, the polystyrene having an average molecular weight (weight average) between 130,000 and 180,000 and its molecular weight distribution curve having a steeply falling high molecular flank. The expandable polystyrene particles are produced by polymerizing styrene in aqueous suspension in the presence of blowing agents with the addition of chain transfer agents or styrene oligomers.

Description

The invention relates to polystyrene containing blowing agent with improved expandability, which is characterized by its molecular weight distribution, and to processes for its production and use.

For the production of polystyrene foams, expandable polymer particles containing foaming agents are known to be heated to temperatures above their softening point until they are foamed into a loose mass with the desired bulk density. This process is called "pre-foaming".

It is desirable that the polymer particles containing the blowing agent dissolve as quickly as possible, i.e. allow to foam at low throughput with high throughput. The expandability of the polymer particles can be improved by conventional low molecular weight plasticizers, but they have the disadvantage that they adversely affect their dimensional stability when the pre-expanded particles are foamed into foam molded articles.

According to DE-A 19 39 696 the expandability of polymer particles from styrene / acrylonitrile copolymers can be improved if the molecular weight is increased by chain transfer agents, e.g. Iodine or dodecyl mercaptan, decreased. In the examples, the chain transfer agent is added at the beginning of the suspension polymerization in the preparation of the polymer particles. A similar process is described in Examples 8 and 9 of DE-A 15 95 627. It has been shown that this procedure produces a polymer with a very high residual monomer content, which shrinks considerably when foamed and therefore cannot be processed into usable moldings.

The invention was based on the object of improving the expandability of particulate polystyrene containing blowing agent so that it can be foamed at high throughput rates to low densities without the dimensional stability of foam molded articles produced therefrom being adversely affected.

This object is achieved by the particulate propellant-containing polystyrene according to the invention.

The blowing agent-containing polystyrene particles according to the invention differ from those according to the prior art in their molecular weight and their molecular weight distribution. The molecular weight is determined by gel permeation chromatography. The GPC method is described in detail in G. Glöckler, Polymer Characterization, Chromatographic Methods Volume 17, Hüthig-Verlag, Heidelberg 1982. According to the invention, the polystyrene has an average molecular weight M _W (weight average) between 130,000 and 180,000, preferably between 140,000 and 175,000 and in particular between 142,000 and 172,000. It is also characterized by its molecular weight distribution, the high molecular flank of the molecular weight distribution curve having to be so steep that the difference in the mean values (M _{Z + 1-} M _Z ) is less than 150.10 ³ , preferably less than 130.10 ³ and in particular less than 120.10 ³ . The averages mentioned are described and defined by HG Elias, Makromolekule, Hüthig-Verlag 1971, pages 52 and 64. The figure shows a typical GPC measurement. Molecular weight distribution curve in which the relative frequency n _{rel of} the polymer chains of a certain molecular weight is plotted against the molecular weight. M _w is the weight average and M _n the number average; M _Z and M _{Z + 1} are further mean values which can be determined by calculation according to Elias and which lie on the high molecular flank of the curve. Their difference (M _{Z + 1} -M _Z ) is a measure of the steepness of this part of the curve.

The polystyrene particles contain one or more blowing agents in a homogeneous distribution. Suitable blowing agents are e.g. under normal conditions gaseous or liquid hydrocarbons or hydrogen halides which do not dissolve the styrene polymer and whose boiling point is below the softening point of the polymer. Suitable blowing agents are e.g. Propane, butane, pentane, cyclopentane, hexane, cyclohexane, dichlorodifluoromethane and trifluorochloromethane. The blowing agents are generally present in the polystyrene particles in amounts of between 3 and 12% by weight, preferably between 5 and 8% by weight, based on the polystyrene.

The polystyrene particles can also contain additives to other substances that give the expandable products certain properties. Flame retardants based on organic bromine or chlorine compounds, such as tris dibromopropyl phosphate, hexabromocyclododecane, chlorinated paraffin and synergists for flame retardants such as dicumyl and highly decomposable organic peroxides; also antistatic agents, stabilizers, dyes, lubricants, fillers and substances which have an anti-adhesive effect when pre-foaming, such as zinc stearate, melamine-formaldehyde condensates or silica, and agents for shortening the mold release time when foaming, such as glycerol esters or hydroxycarboxylic acid esters. The Depending on the intended effect, additives can be distributed homogeneously in the particles or can be present as a surface coating.

According to a preferred process, the blowing agent-containing polystyrene particles according to the invention are produced by polymerizing styrene in aqueous suspension, the blowing agents described above being added before, during or after the polymerization. During the polymerization, with a styrene conversion between 20 and 90%, 0.01 to _0. 5 wt.%, Preferably 0.05 to 0.3 wt.% Of a bromine-free organic compound having a chain transfer constant K between 0.1 and 50 admitted.

It is known that chain transfer agents or regulators reduce the molecular weight of styrene polymers. This is used in the production of polystyrene with improved flowability, with 0.01 to 0.05% by weight of a regulator, e.g. Adds dodecyl mercaptan. This simple procedure does not achieve the goal with expandable polystyrene: If the chain transfer agents are added here before the polymerization, blowing agent-containing polystyrene particles are obtained, which shrink considerably after expansion and result in foam molded articles with low dimensional stability during sintering.

According to DE-A 24 48 476, bromine compounds are added towards the end of the suspension polymerization in the production of blowing agent-containing polystyrene particles. These cause expanded polystyrene particles with a very fine-celled edge zone to form after foaming. Some of these bromine compounds also act as regulators. If the bromine compound mentioned is added before the conversion of 90% of the styrene polymerization, expanded particles with a consistently fine-cell structure are obtained after foaming, which give foam moldings with far too low dimensional stability and a strong tendency to collapse.

EP-B 9 556 describes finely divided expandable styrene polymers which contain 0.00005 to 0.01% of organic bromine compounds, for example hexabromobutene (2). The addition of the bromine compounds in the polymerization causes fine-cell foams to be formed with low minimum mold retention times. The patent states that when the bromine compounds are used in the claimed low concentrations, there is no discernible influence on the molecular weight of the polystyrene. In addition, it has been shown that when using higher concentrations the molecular weight and the molecular weight distribution can be influenced in a targeted manner, but then that foam with extreme fine-cell structure arise, which have poor dimensional stability and insufficient thermal conductivity.

The suspension polymerization of styrene is known per se. It is described in detail in the plastics manual, volume V, "Polystyrene", Carl Hanser-Verlag, pages 679 to 688. Styrene is suspended in water, with conventional organic or inorganic suspension stabilizers, preferably in amounts between 0.05 and 2% by weight .Z, based on the suspension, are added. The polymerization is generally carried out at a temperature between 80 and 130 ° C., preferably between 100 and 120 ° C. Organic polymerization initiators, e.g. Peroxides or azo compounds are used which decompose into radicals under the influence of heat. They are used in amounts of 0.01 to 1% by weight, based on the monomers.

The chain transfer agents are added to the polymerization batch at a conversion between 20 and 90%, preferably between 40 and 60%. The conversion is the percentage of the polymerized monomers, based on the total monomers used. It can be measured, for example, by stopping the polymerization after a certain time, e.g. by adding inhibitors, and the amount of unpolymerized monomers is determined.

Chain transfer agents with a transfer constant K (according to Vollmert, floor plan of macromolecular chemistry, Springer Verlag 1962, pages 52 and 71) between 0.1 and 50, preferably between 1 and 30, are used. For example:

According to another preferred process, the blowing agent-containing polystyrene particles according to the invention are produced by polymerizing styrene in the presence of blowing agents, 0.1 to 10% by weight of a styrene oligomer being added at a conversion between 0 and 90%.

Styrene oligomers are known. They can be produced, for example, by continuous thermal polymerization of styrene under elevated conditions )shock. Their average molecular weight (number average) is between 500 md 5000, preferably between 800 and 2000.

) The polystyrene particles containing blowing agent according to the invention generally have a diameter between 0.2 and 4 mm. They can be pre-foamed using conventional methods, for example with steam, to give foam particles with a diameter between 0.5 and 2 cm and a density between 5 and LOO gl ^-1 . It turns out that the throughput is increased considerably by the added chain transfer agents or styrene oligomers.

The prefoamed particles can then be foamed using conventional methods to give foam moldings with a density of 5 to 100 gl ^-1 .

• Die Polystyrol-Teilchen werden durch Erhitzen auf 155°C aufgeschmolzen und 5 min lang entgast, wobei das Treibmittel verdampft. Durch Extrusion wird ein Polymerstrang mit einer Länge von 20 mm und einem Durchmesser von 5 mm hergestellt. Die Probe wird in eine Zugapparatur eingespannt, die sich in 150°C heißem Silikonöl befindet. Dort wird sie mit einer Zugspannung von 2.10⁴ Pa auf eine Länge von 300 mm gedehnt. Die Dehnviskosität µ ergibt sich als Verhältnis von Zugkraft zu Dehngeschwindigkeit. Nach dem Aufheben der Zugspannung schrumpft die Probe wieder. Die reversible Dehnung Er ist der natürliche Logarithmus des Verhältnisses Ausgangslänge der Probe zur Länge der Probe nach dem Schrumpfen.

The addition of chain transfer agents or styrene oligomers influences the molecular weight and the molecular weight distribution of the resulting polymers, and thus also their rheological properties, which in turn are responsible for the expandability of the polystyrene particles containing blowing agent and the dimensional stability of foam molded articles produced therefrom. For example, a reduction in the expansion viscosity leads to better expandability and a reduction in the reversible expansion with the same expansion viscosity leads to better dimensional stability. Both properties can be measured on the blowing agent-containing styrene polymer particles directly according to a regulation in "Testing high-polymer materials", Carl Hanser Verlag Vienna, 1977, page 422:

• The polystyrene particles are melted by heating to 155 ° C. and degassed for 5 minutes, during which the blowing agent evaporates. A strand of polymer with a length of 20 mm and a diameter of 5 mm is produced by extrusion. The sample is clamped in a pulling device which is in 150 ° C hot silicone oil. There it is stretched to a length of 300 mm with a tensile stress of 2.10 ⁴ Pa. The expansion viscosity µ is the ratio of tensile force to expansion speed. After the tension is removed, the sample shrinks again. The reversible elongation It is the natural logarithm of the ratio of the initial length of the sample to the length of the sample after shrinking.

The blowing agent-containing polystyrene particles according to the invention, measured by the method described, have an elongation viscosity of less than 4.2.10 ⁶ [Pas], preferably from 1.5 to 3.0.10 ⁶ [Pas] and a reversible elongation of less than 0.95, preferably from 0.6 to 0.9.

The parts and percentages given in the example relate to the weight.

example A. Polymerization

A mixture of 150 parts of water, 0.1 part of sodium pyrophosphate, 100 parts of styrene, 7 parts of n-pentane, 0.45 part of benzoyl peroxide and 0.15 part of t-butyl perbenzoate (as polymerization initiators) was added to a pressure-resistant stirred tank made of corrosion-free steel Stirring heated to 90 ° C.

After 2 hours at 90'C, 4 parts of a 10% aqueous solution of polyvinylpyrrolidone were added.

Then the mixture was stirred at 90 ° C. for a further 2 hours, then at 100 ° C. for 2 hours and finally at 120 ° C. for 2 hours. The chain transfer agents or the styrene oligomers were metered into the stirred kettle after the conversion indicated in the table. The granules obtained with an average particle diameter of 1.5 mm were isolated and dried.

B. coating

100 parts of the expandable polystyrene granules with a pearl fraction between 1.0 and 2.3 mm in diameter were coated with 0.4 parts of glycerol monostearate for 3 minutes by tumbling in a paddle mixer.

C. Processing

The coated EPS beads were prefoamed in a continuous Rauscher stirred pre-foamer with flowing steam to a bulk density of 15 g / 1 + 0.1 g / 1 and the throughput was determined in kg / h.

The products were then temporarily stored for 24 hours and welded into a block in a block form, type Rauscher, by vaporization at a pressure of 1.8 bar for 20 seconds.

The specified values for the shrinkage were based on the difference between the internal mold dimensions at room temperature and the dimensions of the Room temperature-cooled foam blocks calculated about 24 hours old. It is given in percent, based on the internal dimension of the mold. The results are summarized in the table.

D. Molecular weight measurement

The average molecular weights and the molecular weight distribution were determined by the GPC method. In doing so, a combination of columns was put together, which guarantees the best possible separation both in the low-molecular and high-molecular range.

Das Füllmaterial sind LiChrospher und LiChrosorb der Fa. Merck (Silicagele) mit einer Korngröße von 10 um (Ausnahme Si 60 = LiChrosorb).The column combination consists of 5 columns connected in series. With an individual length of L = 300 and an inner diameter D _i = 10 mm, the following silica columns were used:

The filling material is LiChrospher and LiChrosorb from Merck (silica gel) with a grain size of 10 µm (exception Si 60 = LiChrosorb).

The flow rate is 2.8 ml / min, the solvent for the polystyrene particles is tetrahydrofuran.

200 µl of a 0.5% solution were injected at room temperature. A differential refractometer from Knauer, dual detector type 61.00, is installed as a detector. A drop counter is used to check the uniform flow rate.

To convert the measurement values obtained to the molecular weight, a calibration curve is created in parallel from 12 narrowly distributed polystyrene samples from Pressure Chemical Co. with a known molecular weight.

To standardize the column combination used, the broad polystyrene sample No. 706 characterized by the National Bureau of Standards is measured. The following mean values were obtained for the molecular weight of this polystyrene sample:

Claims (5)

l. Particulate, blowing agent-containing polystyrene with improved expandability, characterized in that the polystyrene has an average molecular weight M _w (weight average, measured by the GPC method) of between 130,000 and 180,000, and that the high molecular flank of that measured by the GPC method Molecular weight distribution curve is so steep that the difference between the mean values (M _{Z + 1} - M _Z ) is less than 150.10 ³ . 2. Process for the production of particulate, blowing agent-containing polystyrene by polymerizing styrene in aqueous suspension and adding volatile organic blowing agents before, during or after the polymerization, characterized in that during the polymerization at a conversion between 20 and 90% 0.01 to 0.5% by weight of a bromine-free organic compound with a chain transfer constant K between 0.1 and 50 are added. 3. The method according to claim 2, characterized in that mercaptans, preferably tert-dodecyl mercaptan, are used as chain transfer agents. 4. A process for the production of particulate, blowing agent-containing polystyrene by polymerizing styrene in aqueous suspension and adding volatile organic blowing agents before, during or after the polymerization, characterized in that during the polymerization at a conversion between 0 and 90% 0.1 to 10% by weight of a styrene oligomer with an average molecular weight (number average) between 500 and 5000 are added. 5. Use of the particulate, blowing agent-containing polystyrene according to claim 1 for the production of foam moldings with a density of 5 to 100 gl ^-1 .

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